1,1,1,3,3,3-hexafluoro-2-propanone (i.e., hexafluoroacetone or HFA) is used as a starting material to prepare hexafluoroisopropylidene (HFIP) bridged compounds, which are used as monomers in the synthesis of high performance polymers, specialty coatings and pharmaceutical intermediates. Incorporation of hexafluoroisopropylidene moiety into the polymer chain is known to influence the solubility, processability, oxidative stability and electrical properties.
There are a number of methods to prepare HFA in the literature [Krespan and Middleton, J. Fluorine Chem. Rev. 1967, 1, 145] each having certain limitations. For example, in the halogen-exchange fluorination of hexachloroacetone using anhydrous HF and chromium oxide catalyst, the exchange of last chlorine to fluorine is very difficult and the intermediate pentafluorochloro-2-propanone is highly toxic. Epoxidation of hexafluoropropene to hexafluoropropene oxide followed by isomerization to HFA with liquid HF as a solvent requires the use of expensive corrosion-resistant reactors. Finally, perfluoroisobutylene, used as a starting material to prepare HFA is extraordinarily toxic.
The preparation of HFA from 1,1,1,3,3,3-hexafluoropropane (i.e., CF.sub.3 CH.sub.2 CF.sub.3 or HFC-236fa) has several advantages over other methods. The starting material, HFC-236fa, can be readily prepared in high yield from carbon tetrachloride and vinylidene chloride according to U.S. Pat. No. 5,395,997, and the process is amenable to commercial scale-up. The by-product produced in the oxidation step of this process is water and the process can be operated continuously. The present invention relates to a process for the preparation of HFA which can be economically and ecologically superior to existing processes.
1,1,1,3,3,3-Hexafluoropropane (i.e., HFC-236fa) has an atmospheric life time of 265 years indicating a slow reaction with hydroxyl radicals. It is relatively inert to chlorine and bromine radicals too. For example, Henne et al., [J. Amer. Chem. Soc., 67, 1906 (1945)] have reported that HFC-236fa resists chlorination completely in bright sunlight and bromination of HFC-236fa with elemental bromine at 550.degree.-585.degree. C. [L. H. Beck's Thesis, University Microfilms, Inc., The Ohio State University, 1959, p 23] yielded only small amount of 2-bromo-1,1,1,3,3,3hexafluoropropane (i.e., CF.sub.3 CHBrCF.sub.3). Reaction of HFC-236fa with elemental fluorine is not known. Poor reactivity of HFC-236fa towards chlorine and bromine is attributed to heavy shielding of the hydrogens located on the central carbon of HFC-236fa by two adjacent trifluoromethyl groups. The same shielding effect is expected to prevent any radical attack on those hydrogens including fluorine.
Fluorine is different from other halogens in that fluorine-fluorine bond energy is relatively low and carbon-fluorine and hydrogen-fluorine bond energies are very high. Reactions with fluorine require very low activation energies and fluorine sensitized oxidation and halogenation of unsaturated olefins are known in the literature. Miller and co-workers [J. Amer. Chem. Soc., 1956, p 2793] have accumulated enough evidence to show the role of fluorine as an initiator in the oxidation of trichloroethylene and tetrachloroethylene. The use of fluorine to oxidize fluorine containing compounds such as hydrofluorocarbons and hydrochlorofluorocarbons has not been reported. In an attempt to prepare the titled compound, HFC-236fa was reacted with air and fluorine in a fluidized bed reactor and the reaction yielded the desired HFA and its hydrate in high selectivity and good conversion. The oxidation of HFC-236fa in the presence of fluorine provides a route to producing 1,1,1,3,3,3-hexafluoro-2propanone which is cleaner and cheaper than the existing methods.